Ink jet recording records an image or text on a recording sheet such as a paper sheet, applying various principles. This recording method has advantages in providing a relatively high speed recording, low noise and easy color image formation. There have been problems in maintenance or ink clogging of nozzles in this method, but improvements have been made in ink and device, and this method has been widely applied to various fields such as printers, facsimile and computer terminals.
It is required for ink recording sheet to provide printing dots with high density and bright image tone, to provide rapid ink absorption property producing no ink diffusion or blur in overlapped printing dots, and to provide printing dots with smooth periphery and no blurring in which printing ink is not so greatly diffused.
In the recording sheet slow in ink absorption, two or more kinds of color ink drops repel one another at overlapped ink recording portions on the sheet, resulting in image unevenness, or different color inks at different but nearest ink recording portions on the sheet are diffused and mixed, resulting in deterioration of image quality. Therefore, a recording sheet having high ink absorption property is eagerly sought.
Many techniques have been proposed in order to solve the above described problems.
There have been proposed, for example, an ink jet recording sheet paper of low size content wetted with surface treatment coating disclosed in Japanese Patent O.P.I. Publication No. 52-53012, an ink jet recording sheet paper comprising a support and provided thereon, an ink absorption layer disclosed in Japanese Patent O.P.I. Publication No. 55-5830, an ink jet recording sheet paper comprising a layer containing non-colloidal silica powder as pigment disclosed in Japanese Patent O.P.I. Publication No. 56-157, an ink jet recording sheet paper comprising an inorganic and organic pigment disclosed in Japanese Patent O.P.I. Publication No. 57-107878, an ink jet recording sheet paper comprising two void distribution peaks disclosed in Japanese Patent O.P.I. Publication No. 58-110287, an ink jet recording sheet paper comprising two upper and lower porous layers disclosed in Japanese Patent O.P.I. Publication No. 62-111782, an ink jet recording sheet paper comprising amorphous cracks disclosed in Japanese Patent O.P.I. Publication Nos. 59-68292, 59-123696 and 60-18383, an ink jet recording sheet paper comprising a fine powder layer disclosed in Japanese Patent O.P.I. Publication Nos. 61-135786, 61-148092 and 62-149475, an ink jet recording sheet paper comprising pigments or fine particle silica each having a specific physical property disclosed in Japanese Patent O.P.I. Publication Nos. 63-252779, 1-108083, 2-136279, 3-65376 and 3-27976, an ink jet recording sheet paper comprising fine particle silica such as colloidal silica disclosed in Japanese Patent O.P.I. Publication Nos. 57-14091, 60-219083, 60-210984, 61-20797, 61-188183, 5-278324, 6-92011, 6-183134, 7-137431 and 7-276789, or an ink jet recording sheet paper comprising hydrated alumina fine particles disclosed in Japanese Patent O.P.I. Publication Nos. 2-276671, 3-67684, 3-215082, 3-251488, 4-67986, 4-263983 and 5-16517.
The above described methods absorb a recording ink in a layer (void layer) having voids, but the void layer does not almost swell during ink absorption. When the void layer absorbs ink, the ink amount is necessarily restricted by the layer thickness itself.
When the void layer of an ink jet recording sheet has a dry thickness of 40 .mu.m, and the solid packed layer is assumed to have a dry thickness of 22 .mu.m, the void layer has a void volume of only 18 ml (40-22) per m.sup.2 of the ink recording sheet. Although depending on recording methods, ink absorption volume may not be sufficient when a maximum ink amount is supplied. When ink absorption volume is insufficient, ink oozes on the recording sheet surface, so that a clear image cannot be obtained.
In order to obtain a void layer with a high void volume, a method of increasing the coating thickness is most effective, but this is disadvantageous not only in a cost increase in its manufacture, but also in curl occurrence or layer brittleness (particularly crack occurrence at low temperature storage or poor layer adhesion to the support) due to coating amount increase.
It is preferable that the void layer has the lowest possible content of a solid component such as a binder, the largest possible void volume, and the least possible dry thickness.
A typical method of increasing the void volume in a void layer is to increase a fine inorganic particle content relative to a hydrophilic binder content in the void layer, and to contain no other components in the void layer, however, in this method there is a problem in that the layer is extremely brittle.
The brittle void layer is likely to separate from the support of an ink jet recording sheet during its handling, during its feeding in an ink jet printer or during its transporting in the ink jet printer, or fine cracks occur on the layer, resulting in a serious problem that the recording ink diffuses along the cracks.
There are various ink jet recording sheets in which an ink absorbing layer without voids absorbs and retains ink due to swelling of the binder contained in the layer. For example, there are known ink jet recording sheets obtained by coating a hydrophilic binder such as gelatin, casein, starch, alginic acid, polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene oxide, polypropylene oxide, carboxymethyl cellulose, hydroxyethyl cellulose, dextran or pullulan on a support.
These ink jet recording sheets have advantages in providing a clear image with high glossiness and high density and in receiving ink by swelling due to ink of the binder used to give high ink absorption. However, the ink jet recording sheets are inferior in ink absorption speed as compared to ink jet recording sheets comprising a void layer, and the former has a disadvantage in that when a large amount of ink is jetted onto the sheet, ink drops repel one another, resulting in blurring.